bathtub vs. cement board for tile

i was wondering if there is any reason to install cement board under the bathtub. it is a plain and simple recessed bath tub on three sides. i thought maybe only if i put an inch of cement board under the apron of the tub it would be enough so it sits on it instead of putting the tub directly on top of subfloor and then cement board outside would raise tiles too high against the tub.